This application is a Continuation of International PCT/FI00/00883 filed Oct. 12, 2000 which designated the U.S. and was published under PCT Article 21(2) in English.
The invention relates to a transfer belt for a paper machine, the transfer belt comprising a base structure, a fibre batt layer attached to the base structure and arranged to face the fibre web, and a polymer matrix arranged at least on the fibre batt layer side to impregnate the fibre batt layer, the fibres batts extending to the surface of the polymer matrix on the belt surface facing the fibre web.
Transfer belts coated with a polymer or those impregnated throughout with a polymer material have been disclosed in various publications, such as U.S. Pat. Nos. 4,483,745; 4,976,821; 4,500,588; and 4,529,643. In addition, such belts have been described in Finnish Patents 64959 and 64960.
This kind of a transfer belt is typically made by coating a conventional support structure with a polymer material, or by filling the fabric structure entirely with the polymer material. It is also known to impregnate so-called paper machine felt, i.e. to needle a fibre batt layer onto a woven structure, with a polymer material.
A transfer belt is used for transferring the fibre web for example from a press felt or a press fabric forward to a press nip, for transferring it from the press nip onward and finally for transferring the fibre web to another texture or belt. The transfer belt can also be used for other purposes in the paper machine to transfer the fibre web from one process stage to another. A typical feature in these applications is that the fibre web follows more easily a surface to which the force caused by water contained in the fibre web best attaches the web. Therefore the fibre web follows most easily a substantially smooth surface impermeable to water and/or air. An essential problem is that it is difficult to detach the fibre web from this kind of known surface structure, particularly when the fibre web is still wet.
It is an object of the present invention to provide a transfer belt which has suitable surface properties allowing the fibre web to be detached from the belt in a desired manner and ensuring, at the same time, an advantageous transfer belt behaviour during the pressing stage.
The transfer belt of the invention is characterized in that the transfer belt surface facing the fibre web is provided with hydrophilic and, correspondingly, hydrophobic areas and that the hydrophilic and hydrophobic areas are formed by providing the fibre batt layer of the transfer belt with at least two fibres having different surface properties.
An essential idea of the invention is that the transfer belt surface facing the fibre web is made of a fibre layer impregnated with a polymer and comprising fibres of different surface properties. The fibres may differ from one another with respect to their polarity, hydrophilicity, electric charge, surface energy, friction properties or porosity, the transfer belt surface being thus provided with areas having different properties. Another essential idea of the invention is that the surface is ground to be suitably smooth, the fibres on the surface maintaining, however, a certain micro-roughness on it. This roughness can be controlled not only by the roughness of the abrasive means but also by the degree of fineness of the fibre. Hence, when the transfer belt is subjected to compression, the surface becomes smooth and the water included in the fibre web forms a film which spreads evenly onto the surface. Correspondingly, when the compression ceases, the micro-roughness of the surface is restored and the water film breaks into drops. The water then enters the hydrophilic areas and leaves the hydrophobic areas. As a result, the fibre web is no longer firmly attached to the transfer belt, but it can be easily detached from it.